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Boxwood Care: How To Identify And Treat 4 Common Pests and Diseases

A garden staple for centuries, boxwood instantly infuses a landscape with structure and elegance. Its dense, evergreen foliage can be sheared into almost any shape imaginable. For those of us on the East Coast, the fact that deer avoid it only enhances its appeal. There’s just one problem: it’s plagued by a slew of pests and diseases.

ONE BIG FAMILY

Most of us know boxwood as a shrub, but in fact the family is much broader. It includes five genera of trees, shrubs and herbs and around 247 flowering species. The species most familiar to Westerners is Buxus sempervirens, also known as Common box.

In America, Buxus is called boxwood.

Boxwood fleur-de-lys at George Washington’s Mount Vernon

And in England it is called box.

Formal English garden with box

Boxwood has served as hedging and edging in gardens since the 16th century. Today, it is also a common foundation planting. And boxwood topiary, a practice that began in Roman times, is still practiced in great gardens across the world, including Prague’s lovely Vrtba Garden.

Boxwood edging at Paris’ Luxembourg Gardens

HOW TO GROW BOXWOOD

Boxwood grows in full sun or partial shade and once established, is relatively drought tolerant. Like all plants, it prefers well drained soil. Heavy or compacted soils that retain water easily cause the roots to deteriorate. If you want your shrubs to really flourish, make sure your soil is on the alkaline side, or a pH of 7.1 or higher.

Due to boxwood’s preference for alkaline soil, experts generally recommend that you avoid planting it close to azaleas and rhododendrons. The soil for these shrubs should be acid, or somewhere between a pH of 4.5 to 6.0.

Boxwood and azaleas have different soil requirements.

BE MINDFUL OF THE ROOTS

According to Lynn Batdorf, former curator of the U.S. Arboretum’s National Boxwood Collection and internationally-recognized expert on everything boxwood, a boxwood rootball is shaped like a pancake. Even on the biggest shrubs, the roots typically extend no deeper than a foot. However, boxwood surface roots can extend many feet behind the drip line. As a result, it’s important to give the shrubs lots of room to grow.

This includes avoiding planting anything directly underneath or around them. And it’s important not to site shrubs close up to impermeable materials like walls or pathways that will inhibit their growth. If you must plant in a boxwood’s vicinity, Batdorf suggests planting bulbs so that you only disturb the roots once.

This looks nice, but is actually not good for your boxwood.

To encourage healthy growth, feed your boxwood in early spring with a balanced fertilizer high in nitrogen. Keep the soil cool and protect the roots by spreading a 1 to 2-inch layer of pine bark mulch around the plant base. (Batdorf recommends pine bark because it’s lighter than hardwood, which can suffocate the roots.)

Make sure to leave a 6-inch distance from the trunk to prevent any moisture from building up that might encourage root rot.

AN ANCIENT PLANT WITH MODERN DAY DISEASES

Although generally viewed as low maintenance (some specimens have lived for over 400 years), boxwood is nonetheless saddled with its own set of pests. Most recently, boxwood blight has become a significant problem. First seen in England in the 1990s, blight has traveled overseas and is now decimating landscapes across America.

BOXWOOD BLIGHT

What is blight? Blight is caused by the fungal pathogen, Calonectria buxicola, which produces leaf spots, stem cankers, defoliation and eventual death of vulnerable plants. The disease starts with an initial leaf spot that resembles a black circle with red halo.

As it spreads, blight infects entire leaf surfaces, causing the foliage to drop from the plant. Eventually stems are infected, too, forming black lesions or cankers. Once the cankers girdle the boxwood stem, they cut off the water supply and the branch dies. Eventually, deprived of its sources of energy, the plant dies from exhaustion.

Due to blight, many people have switched from the disease-prone English box to the more resistant Japanese and Korean boxwood species. Many of these new varieties are drought-tolerant, have good disease resistance and keep their dark green color all winter. For a more in-depth article on boxwood blight and ways to deal with it, click here for tips from international boxwood expert and former curator of the U.S. Arboretum’s National Boxwood Collection, Lynn Batdorf.

Treatment: If you suspect blight, first get a diagnosis from a trained professional. Start by pruning out diseased stems, making sure to sterilize all clothing and equipment. Then rake up and dispose of the foliage. (Some people even vacuum.) Last resort would be to apply fungicides such as chlorothalonil or chlorothalonil mixed with thiophanate methyl. These have shown some promise in controlling this disease, although they must be applied every few weeks throughout the growing season.

Updated 7/28/2018 Many professionals are now recommending the removal of the entire plant, as well as the soil around it to prevent the spread of the pathogen.

BOXWOOD DECLINE

Another common disorder of both American and English box is boxwood decline. This is usually caused by a combination of diseases brought on by poor-drainage, poor air circulation and occasionally weather.

Appearing before new growth occurs in the spring, the fungal disease Volutella causes leaves on the tips of infected branches to turn red, then bronze and finally yellow. In moist areas, the fungus will sometimes produce small pink-colored fruiting bodies. Infected branches die back.

The fungal disease Macrophoma also causes leaf spot and straw-colored leaves. However, it is easily distinguished from Volutella by its numerous black fruiting bodies.

Fungal damage. Photo credit: University of Maryland

Treatment. The best way to control for fungal diseases is to thin your shrubs regularly. This will increase air-circulation within the bush. Do this by reaching down into the plant and pruning out handfuls of stems until light can penetrate into the center of the shrub. Prune only when the foliage is dry to prevent the spread of mold spores. Afterwards, rake up and dispose of all infected leaves and branches.

BOXWOOD LEAFMINER

You might have noticed that your boxwood leaves have little red spots and appear puffy. This is the work of Monarthropalpus flavus, or Boxwood Leafminer. The tiny orange insect can be found buzzing around plants in the spring when new growth appears, inserting its eggs inside the leaf membrane. The larvae don’t hatch until autumn, however, when they begin mining inside the leaf. This causes the leaves to turn orange and eventually drop off.

Leaf Miner damage. Photo credit/Cornell University

Treatment: If you observe insects swarming around your shrubs, treat them with a systemic insecticide applied to the foliage in April or May. Or, apply granular systemic insecticide to the soil around the trunks in early spring.

Boxwood Psyllid

Boxwood psyllids are small insects that cause new leaves to cup as the nymphs extract sap from the tender foliage. Damage is especially noticeable on American box. Psyllids may affect the looks of the plant, but unlike leaf miners, they are seldom a threat to the overall health of the shrub.

Treatment: Psyllid damage is more a question of aesthetics than anything else and will produce only scattered cupping. Prune and dispose of infected limbs in mid-May before nymphs become adults and have a chance to lay eggs.

About carole funger

I'm a garden designer and Maryland Master Gardener living in the Washington, DC area. I blog about new trends in horticulture, inspiring gardens to visit and the latest tips and ideas for how to nurture your own beautiful garden. Every garden tells a story. What's yours?